Spray pyrolysis process for preparing superconductive films
BACKGROUND OF THE INVENTION 
     The present invention relates to an improved method for preparing high temperature superconductive films and, specifically, to an improved spray pyrolysis method for preparing Bi-Sr-Ca-Cu-O superconducting films. 
     Spray pyrolysis, a low cost method for preparing thin films, is well known For example, U.S. Pat. No. 4,327,119 to Lis, et al. describes a spray pyrolysis method for synthesizing and producing thin films. The films of Lis, et al. find particular utility in photovoltaic devices, and particularly, in solar heat absorbing panels Similarly, U.S. Pat. No. 4,336,285 to Squillante relates to an improvement on the Lis, et al. method which involves the identification and use of certain highly soluble organic acids as reducing agents in the spray pyrolysis solution. Again, the suggested use is for photovoltaic devices. 
     Spray pyrolysis has been recognized also as a useful method for applying and preparing superconducting thin films. Maki Kawai, et al. describe the formation of Y-Ba-Cu-O superconducting films using a spray pyrolysis method. Of particular relevance, Hitoshi Nobumasa, et al. describe the formation of Bi(Pb)-Sr-Ca-Cu-O superconducting films using a spray pyrolysis application method. The techniques involve relatively low temperature deposition of chemical precursors, such as metal nitrates and acetates, onto suitable substrates, followed by higher temperature processing, both to convert the precursors to oxides and to induce the formation of the superconducting phase. According to the Nobumasa method, an aqueous solution of Bi, Sr, Ca and Cu nitrates was prepared. A lead doping agent was added as a substitute for a portion of bismuth. The solution was sprayed onto a heated MgO substrate. Thereafter, the sprayed substrate was air-cooled and then reheated to 845° C. After a select heating time of two hours or more, the substrate again was allowed to air cool to room temperature. 
     While the known pyrolysis methods for applying thin superconductive films have yielded useful high quality films, the films often exhibit high porosity and gross textures. These properties are the result of volume shrinkage and the evolution of gaseous byproducts. 
     In the literature concerning the processing of bulk superconducting oxides, two methods have been reported for enhancing the densification of Bi-Sr-Ca-Cu-O materials. The first is a melt-quench technique. This method is described by T. Komatsu, et al. in (1) J. Appl. Phys, 27, L533 (1988) and (2) J. Appl Phys. 26, L1148 (1987). The second method is a flux-sinter method in which materials, such as potassium fluoride, are added to the spray solution to enhance mass transport. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an improved spray pyrolysis method for preparing thin superconductive films. 
     Another object of the invention is to provide an improved spray pyrolysis method which reduces the porosity of the resulting superconductive films and which produces a smoother film texture. 
     Yet another object of the present invention is to apply bulk superconducting densification techniques to thin superconducting films. 
     Thus, in accordance with one aspect of the present invention, there is provided an improved spray pyrolysis method for preparing thin superconductive films. The method, referred to herein as the &#34;melt-quench method&#34;, comprises the steps of preparing a spray pyrolysis solution of Bi, Sr, Ca and Cu salts in a solvent; heating a substrate to a first temperature, preferably between about 400° and 600° C.; spraying the solution onto the heated substrate to form a film on the substrate; heating the film and substrate at a temperature between about 700°-825° C. for between about 30 and 60 minutes; and thereafter, further heating the film and substrate in an apparatus set at a temperature between about 900° and 1000° C. Once the film and substrate reach a temperature of about 880° C., they are immediately removed from the apparatus and placed in a further heat treatment zone where they are heat treated at a temperature between about 840° and 860° C., preferably about 855° C. for about one hour. The film and substrate are then removed from the heating apparatus and allowed to cool to ambient temperature. 
     In accordance with another aspect of the present invention, there is provided another method for preparing high temperature, thin, superconductive films using a spray pyrolysis process According to this method, which is referred to herein as the &#34;flux-sinter method&#34;, a fluxing agent is added to the solution. The fluxing agent preferably is sodium nitrate or potassium fluoride The solution is sprayed onto a heated MgO or BeO substrate and, thereafter, the film and substrate are heated to a temperature between about 700°-825° C., preferably about 800° C., for 30-60 minutes, as in the melt-quench method. In contrast to the melt-quench method, however, the melt heating step is omitted. This melt heating step is the heat treating step performed at about 880° C. In the flux-sinter method, after the 700°-825° C. heat treatment, the film and substrate are heat treated at about 840°-860° C., preferably about 855° C., as in the final melt-quench heat treatment step. 
     As a result, two simple film densification processes result which are used in conjunction with low cost pyrolysis techniques to prepare dense oriented superconducting films. Namely, the methods produce Bi-Sr-Ca-Cu-O films having a critical temperature (T c ) of 81-83K and critical current densities of at least about 4000 A/cm 2  at 78K. 
     Also, a further heat treating, or annealing, step can be performed on the film after it has cooled to ambient temperature. In accordance with this additional annealing step, the film and substrate are reheated at a temperature of about 600°-700° C. for about 2-15 hours in an inert gas environment. As a result, the T c  of the film is enhanced to about 87K. 
     These and other objects, features and advantages of the present invention will be more readily apparent from a review of the detailed description of preferred embodiments, when considered with the figures of drawing, a brief description of which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1(a) presents a photomicrograph of the superconductive film structure of the present invention made by the melt-quench method. FIG. 1(b) presents a photomicrograph of the superconductive film structure of the present invention made by the flux-sinter method. 
     FIG. 2 is a graph of resistivity vs. temperature for superconductive film structures of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In performing the method of the present invention, the first step involves the preparation of an aqueous solution. Bismuth, strontium, calcium and copper are dissolved in a solvent. The metals can be in the form of oxides, carbonates, nitrates and the like. A preferred group of starting materials includes bismuth oxide, strontium carbonate, calcium carbonate and cupric oxide. Preferred stoichiometric amounts of Bi:Sr:Ca:Cu are 2:2:1:2 and 4:3:3:4. 
     The solvent can be selected from a variety of well-known solvents, the important criteria being that the solvent produces a uniform, homogeneous solution when mixed with the solutes. Concentrated nitric acid is preferred when bismuth oxide, strontium carbonate, calcium carbonate and cupric oxide are used. 
     The aqueous solution should have a cation concentration of at least about 0.1 M. The cation concentration should be limited at the upper end to avoid precipitation of the dissolved materials and to avoid a highly acidic solution which could attack the substrate. Preferably, the molar cation concentration should be in the range from about 0.1 M to 1 M. 
     Once the desired solution is obtained, it is sprayed onto a substrate. Any of the well-known spray pyrolysis techniques may be used for solution application. The substrate is either a magnesium oxide, preferably a single crystal [100]magnesium oxide, or a hot-pressed beryllium oxide substrate. Beryllium oxide substrates are particularly preferred for high frequency applications due to its favorable dielectric constant, thermal conductivity, and microwave properties. 
     The substrate is heated to a temperature between about 400° and 600° C. Below 400° C., the final structure may exhibit cracking due to evaporation of trapped water not otherwise removed due to the low substrate temperature. As the temperature of the substrate exceeds 600° C., the risk increases that uniform film distribution will be sacrificed due to aerosol evaporation before it strikes the substrate. Preferably, the substrate temperature is maintained at about 600° C. 
     After spray distribution, the film undergoes a first heat treatment. The film is heated to a temperature between about 700°-825° C. During this heat treatment step, the water present in the film is removed and the requisite metal oxides are produced. The upper temperature limit reflects the fact that, at temperatures above this limit, the metal nitrates that are present may attack the substrate. Preferably, the temperature is about 800° C. 
     The length of this first heat treatment step depends on the temperature selected. Generally, the heat treating should continue for about 30 to 60 minutes, with the longer time necessary at the lower temperatures. 
     After this first heat treatment, the film is moved into a hotter zone, preferably a zone at a temperature between about 900°-1000° C. During this step, the high temperature melts the film. It is desirable to have the melt occur as fast as possible; however, at temperatures much in excess of 1000° C., a loss of control of the substrate temperature occurs due to thermal gradient differences between the thermocouple and the substrate. The temperature of the heat zone is preferably about 950° C. 
     The film temperature is not allowed to rise to the temperature of the hotter heating zone, however. Instead, the film and substrate are removed immediately upon reaching a temperature in the range of about 870°-890° C., particularly about 880° C. Generally, only about 10 seconds are required to heat the film to 880° C. The time depends on the thermal mass of the devices, such as substrates, substrate holders, and thermocouples, in the hot zone. It will vary from system to system. 
     The film and substrate are removed from the hot zone to a third heat treatment zone corresponding to the phase transformation temperature of the film. This heat treatment step allows a recrystallization of the Bi-Sr-Ca-Cu-O film. The temperature is between about 840°-860° C., preferably about 850° C. The film should remain in this zone for at least 30 minutes, preferably at least one hour. 
     Thereafter, the film and substrate are removed from the heat treating zone and allowed to cool to ambient temperature. The process is carried out in an air environment to provide the necessary oxygen for oxidation. 
     According to another method of the present invention, a densified superconducting film can be achieved by the application of a flux-sinter method. According to this process, a fluxing agent is added to the aqueous solution. The fluxing agent improves the mass transportation properties of the film. The fluxing agent should have a low boiling point to insure that it will be removed from the final film product by the various heat treatment steps. Two preferred agents are sodium nitrate and potassium fluoride. 
     The amount of the fluxing agent varies according to application. Preferably, the amount of the agent equals about 1/7 to 1/12 on a molar base of the total metal ion concentration of the Bi-Sr-Ca-Cu solution, preferably about 1/10 of the total metal concentration. 
     The resulting aqueous solution is applied to the substrate in a manner identical to the melt-quench method. Likewise, the resulting film and substrate undergo the same heat treatment as described above with respect to the melt-quench process, with one major exception. Specifically, the film and substrate do not undergo the melt densification step, i.e., the 10 second, 870°-890° C. heat treatment step. 
     The films produced by both methods are similar. Both methods produce dense superconducting films exhibiting a high degree of crystallographic orientation with the c-axis normal to the substrate surface on both single crystal MgO and polycrystalline BeO substrates. FIG. 1(a) and 1(b) show two scanning electron microscope (SEM) photographs of the surface morphology of films made by (a) the melt-quench method, and (b) the flux-sinter method. The resistance versus temperature plots for samples prepared on both MgO and BeO substrates by either of the methods generally were very similar, with zero resistance being attained at 81-83K. FIG. 2 is a representative plot. Critical current densities of at least 4000 A/cm 2  at 78K were also attained. 
     In a further embodiment of both methods, an additional reheating, or annealing, step is performed. According to this step, the film is reheated to about 600°-700° C. for about 2-15 hours in an inert environment. Argon is a preferred inert gas. At temperatures much above 700° C. the superconducting phase becomes unstable. As a result of this additional annealing, an enhancement of T c  occurs, as shown in FIG. 2(b). As shown, the T c  is increased to about 87K. 
     The present invention will be illustrated further by the following examples. These examples are meant to illustrate and not to limit the invention, the scope of which is defined solely by the appended claims. 
     EXAMPLE 1 
     A spray solution was prepared by dissolving bismuth oxide, strontium carbonate, calcium carbonate and cupric oxide in concentrated nitric acid. The metal stoichiometry was 2:2:1:2 (Bi:Sr:Ca:Cu). The solution then was diluted to yield an aqueous solution that was 1.6 M in nitric acid and had a total metal ion concentration of 0.5 M. The solution then was sprayed onto a BeO substrate using a glass concentric nebulizer. A 20 micron film was applied to the substrate. The substrate was held in place on a 600° C. heated block. 
     Thereafter, the film was placed in a fused quartz combustion tube positioned in a single zone furnace. An air environment was present. The sample temperature was measured by a thermocouple fixed on a sample holder. 
     The film first was heated to 800° C. for 30 minutes, then moved into a hotter zone set at 950° C. When the sample temperature reached 880° C. (about 10 seconds), the film was removed immediately and placed in a further heat treatment zone set at 855° C. for one hour. The film then was removed and allowed to air cool to ambient temperature. 
     The resulting film had a critical temperature (T c ) of 81K and a critical current density of 4000 A/cm 2  at 78K. 
     EXAMPLE 2 
     The spray deposition apparatus and spray solution used in Example 2 were identical to that described in Example 1. Several samples of Bi-Sr-Ca-Cu-O films were deposited on a substrate and heat treated, varying the final anneal temperature and gas environment. The annealing time was one hour. The results are summarized in Table 1 below. 
     
                       TABLE 1                                                     
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      Substrate Temp.                                                     
                   Anneal              Film                               
Sample                                                                    
      at Melting Step                                                     
                   Temp.   Environ-                                       
                                  T.sub.c                                 
                                       Thickness                          
#     (°C.) (°C.)                                           
                           ment   (K)  (μm)                            
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1     880          865     Air    70   3                                  
2     880          860     Air    80   5                                  
3     880          855     Air    81   3                                  
4     880          850     Air    83   4                                  
5     880          845     Air    82   3                                  
6     880          840     Air    80   3                                  
7     880          835     Air    73   5                                  
8     880          850     Oxygen 76   4                                  
9     880          850     Argon  none --                                 
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     EXAMPLE 3 
     A spray solution was prepared by dissolving bismuth oxide, strontium carbonate, calcium carbonate, cupric oxide and potassium fluoride in concentrated nitric acid. The metal stoichiometry was 2:2:1:2 (Bi:Sr:Ca:Cu) and the concentration of potassium fluoride was 1/10 of the total anion concentration on the molar base. The solution then was diluted to yield an aqueous solution that was 1.6 M in the nitric acid and had a total anion concentration of 0.5 M. The solution then was sprayed onto a MgO substrate using a glass concentric nebulizer. A film of 25 microns was applied to the substrate. The substrate was held in place on a 600° C. heated block. 
     Thereafter, the film was placed in a fused quartz combustion tube positioned in a single zone furnace. An air environment was present. The sample temperature was measured by a thermocouple fixed on the sample holder. 
     The film first was heated to 800° C. for 30 minutes, then moved into a hotter zone set at 850° C. and allowed to anneal at this temperature for one hour. The film then was removed and allowed to air cool to ambient temperature. 
     The resulting film had a critical temperature (T c ) of 83K. 
     EXAMPLE 4 
     The spray deposition apparatus in Example 4 was identical to that described in Example 3. Several samples of Bi-Sr-Ca-Cu films were deposited on a substrate and heat treated, varying fluxing agent (potassium fluoride or sodium nitrate) concentration, anneal temperature, and anneal time. The annealing environment was air. The results were summarized in Table 2. 
     
                       TABLE 2                                                     
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        Fluxing Agent                                                     
        Concentration                                                     
                    Anneal                                                
Sample  (per total anion                                                  
                    Temp.     Anneal T.sub.c                              
#       concentration)                                                    
                    (°C.)                                          
                              Time   (K)                                  
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1       1/6         850       1 hour 60                                   
2       1/7         850       1 hour 78                                   
3       1/8         850       1 hour 80                                   
4       1/10        850       1 hour 83                                   
5       1/12        850       1 hour 80                                   
6       1/15        850       1 hour 40                                   
7       1/10        860       1 hour 79                                   
8       1/10        840       1 hour 46                                   
9       1/10        850       17 hour                                     
                                     82                                   
10      1/10        850       .5 hour                                     
                                     78                                   
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     EXAMPLE 5 
     The Bi-Sr-Ca-Cu-O film of Example 1 was further treated by reheating the sample in argon to 700° C. for 2 hours. The resulting film evidenced a T c  of 87K. 
     EXAMPLE 6 
     The Bi-Sr-Ca-Cu-O film of Example 3 was further treated by reheating the sample in argon to 700° C. for 2 hours. The resulting film evidenced a T c  of 87K. 
     EXAMPLE 7 
     Several samples of Bi-Sr-Ca-Cu-O films with high T c  were further heat treated in argon, varying the anneal time and temperature. The samples were produced by both the melt-quench and the flux-sinter methods. The results are summarized in Table 3. 
     
                       TABLE 3                                                     
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                 Anneal                                                   
Sample  Original Temperature  Anneal Final                                
#       T.sub.c (K)                                                       
                 (°C.) Time   T.sub.c                              
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1       81       800          2 hour none                                 
2       81       700          2 hour 87K                                  
3       81       600          2 hour 86K                                  
4       81       500          2 hour 81K                                  
5       81       700          15 hour                                     
                                     87K                                  
6       82       700          .5 hour                                     
                                     84K                                  
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     The present invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While presently preferred embodiments of the invention have been described for the purpose of disclosure, numerous changes in the details of construction and arrangement of parts may be made without departing from the spirit of the present invention and the scope of the appended claims.